The present invention is directed to UV curable catalyst compositions and methods of depositing an ultra-thin metal or metal alloy layer on a substrate. More specifically, the present invention is directed to UV curable catalyst compositions and methods of depositing an ultra-thin metal or metal alloy layer on a substrate where the UV curable catalyst has high surface area particles.
Many industries where workers desire to coat or form one or more metal or metal alloy layers on substrates employ catalysts. Such catalysts are employed in electroless deposition of metal or metal alloys. Electroless deposition or plating is based on the presence of a chemical reducing agent being added to the deposition bath. Such chemicals supply electrons to substrate metals, which transmit the electrons to the positively charged metal ions in the bath reducing these ions to metal in the same manner in which electric current reduces metal ions to metals in electrolytic or electrodeposition baths.
Electroless plating produces several desirable results. Workers often have difficulty in depositing metal layers of uniform thickness on substrates with crevices or holes using electrolytic methods of plating. This attribute is important in many industries such as in the electronics industry, in which printed circuit or printed wiring boards demand uniform metal deposits plated into high aspect-ratio through-holes. Other properties and applications of electroless plating are deposits which may be produced directly upon nonconductors, deposits in which are often less porous than electrolytic plating, and also deposits which often have unconventional chemical, mechanical or magnetic properties (such as higher hardness and wear resistance).
Another attribute of electroless plating is that the process is auto-catalytic and deposition occurs on a catalytic surface. Accordingly, a catalyst is required. Catalysts employed in electroless metal deposition vary widely in composition depending on the metal or metal alloy to be deposited as well as the use of the article made. In addition to the manufacture of printed wiring boards, electroless plating using catalysts are employed in the manufacture of various decorative articles, and in numerous other electronic applications such as in the formation of electromagnetic interference (EMI) and radio frequency interference (RFI) shielding.
EMI radiation is created by operation of many diverse forms of electronic equipment ranging from microwave equipment to home computers. The radiation occurs because electronic devices emit “noise” in a frequency range of 60 Hz to more than 1000 MHz, and is picked up by other devices or by conduction through power lines that act as antennas. EMI radiation may interfere with other devices and has been known to cause such diverse problems as interference with police mobile radios, communication systems, scientific test equipment and cardiac pacemakers.
One approach to limiting electromagnetic containment is the use of an EMI shield to contain the radiation. Containment requires special shielding materials, components, and structures, which prevent generated energy from escaping and acting as a source of disturbance.
Effectiveness of electromagnetic containment is determined by the degree to which the field strength is attenuated as a result of reflection or absorption by the shielding material. Shielding efficiency is calculated as a logarithmic function of the ratio of unshielded EMI transmission to shielded EMI transmission and is expressed in decibels (db). Because of its logarithmic nature, an increase of 30 db in shielding efficiency for a given wavelength or frequency of electromagnetic radiation represents a 1000% increase in the shielding efficiency of a coating. A coating with a shielding efficiency of 30 db, for example, eliminates 99.9% of the total EMI radiation. A 60 db coating eliminates 99.9999% of the total EMI radiation.
A number of different shielding methods have been used commercially. One method involves applying a metallic coating over a plastic housing for electronic devices. Such methods include galvanic deposition, spray coating such as by arc-spraying or spraying the metal on as a paint, cathode sputtering, chemical metallizing and vacuum metallizing. Metal coatings have included copper, silver, chromium, nickel, gold and zinc. Such methods have suffered from a number of deficiencies such as macro or microscopic cracking, peeling of coatings, limited shielding effectiveness, oxidation of metals in the coatings, distortion of thermoplastic substrates, and expensive application equipment.
A more suitable method of forming an EMI shield has been by electroless deposition of a metal on the non-conductive housing materials. Electroless deposition of non-conductors such as plastics involved immersing a part in a series of aqueous baths, which both prepare the surface of the part for deposition and permit metallization. Following conventional pretreatment steps, the part is then immersed into a catalyst containing noble metals, such as a colloidal tin/palladium catalyst, to render non-conductive surfaces catalytic to deposition of the desired plating metal. Following catalysis, the part is then immersed into an electroless plating bath containing dissolved metals which, in contact with the plating catalyst, results in deposition of a coating of the metal onto the catalyzed surface.
While the foregoing electroless catalyst and method was superior to many of the earlier methods employed to address the problem of EMI shielding, the electroless coating process was not selective. The entire part was immersed into the colloidal catalyst solution followed by immersing the part into a metal plating solution. The result was that metal was plated over the exterior as well as the interior surface of the non-conductor part. Where aesthetics are important in the marketing of electronic components, an exterior metal coated housing for the electronic component is undesirable. Typically, the industry paints the housing. This is a time consuming and wasteful step, especially where housings are most often molded in a desired color.
U.S. Pat. No. 5,989,787 discloses an alkaline, hydrophilic activating catalytic solution for selectively electroless plating of metals on a non-conductive substrate. The solution is composed of a mixture of copper lactate or zinc lactate, a palladium salt, such as palladium chloride, and an alkaline medium. The electroless plating method includes the steps of applying the hydrophilic activating catalytic solution on a substrate to form a photosensitive film on the substrate, and selectively exposing the photosensitive film to UV (ultra-violet) radiation or scanned by laser rays to deposit palladium catalyst on the substrate, developing away any un-exposed photosensitive film, and electroless plating the substrate using the palladium catalyst as an activating catalyst.
When the photosensitive film is exposed to UV radiation or scanned by laser rays, copper ions or zinc ions from the copper or zinc lactate are activated to interact with palladium ions. The palladium ions are then reduced to metallic palladium thereby depositing catalytic palladium on the substrate. The patent states that the formation of palladium lactate is important in the performance of this invention because palladium lactate is soluble in the alkaline environment. As stated in the patent the highly soluble palladium lactate permits a distinct contrast between the radiation exposed area and non-exposed area with a short time exposure to the radiation. Further, because the lactate is not likely to be subjected to hydrolytic decomposition, the un-exposed photosensitive film may be removed with water or water-based liquids without forming unnecessary compounds. One disadvantage of the composition and method disclosed in the '787 patent is the use of only palladium salts for the catalyst. Such salts are typically more expensive than other catalytic salts. Further any undeveloped palladium lactate is lost during the rinse steps resulting in the lose of costly palladium metal.
Another disadvantage of the composition and method of the '787 patent is that it does not disclose methods of adhering palladium to the substrate. Accordingly, in order to prepare the substrate for receiving the palladium, classic chromic etch methods would be used. Such chromic etch methods are undesirable because they are hazardous to workers, and any waste generated in their use is environmentally unfriendly.
Accordingly, there is still a need for an improved composition and method of forming a metal layer on a non-conductive substrate.